Forward masking in different cochlear implant systems

Abstract

The goal of this study was to evaluate, from a psychophysical standpoint, the neural spread of excitation produced by the stimulation of different types of intracochlear electrode arrays: the Ineraid, the Clarion S-Series on its own or with the Electrode Positioning System (EPS), and the Clarion HiFocus-I with the EPS. The EPS is an independent silicone part designed to bring the electrode array close to the modiolus. Forward masking was evaluated in 12 adult subjects (3 Ineraid, 4 Clarion S-Series, 3 Clarion S-Series+EPS, 3 HiFocus-I+EPS) by psychophysical experiments conducted using trains of biphasic stimuli (813 pulses per second, 307.6 micros/phase). Masker signals (+8 dB re: threshold, 300 ms) were applied to the most apical electrode. Probe signals (30 ms, 10-ms postmasker) were delivered to more basal electrodes. Masked and unmasked detection thresholds of probe signals were measured. For both Clarion HiFocus-I subjects, measurements were conducted in both monopolar and bipolar stimulus configurations. No major differences were found in forward masking between the different intracochlear electrode arrays tested in the monopolar configuration at suprathreshold levels equivalent to those used in speech-coding strategies, but significant differences were found between subjects. A significant negative correlation also was found between the level of forward masking and the consonant identification performance. These measurements showed that the neural spread of excitation was more restricted in the bipolar configuration than in the monopolar configuration for HiFocus-I subjects. It was found that CIS strategies implemented without using apical electrodes, which showed high levels of masking, could improve consonant identification.

Implications of Deep Electrode Insertion on Cochlear Implant Fitting

“…They showed that for four out of six subjects, the stimulation of the two most apical electrodes did not produce the perception of pitch differences. Boëx et al (2003) reported the case of one Clarion S-series listener, implanted with a positioner (Fayad et al 2003) who improved his consonant identification performance when his deepest apical electrodes (above 532-) were deactivated. This subject showed also important neural interactions on these apical electrodes.…”

Probing the Electrode-Neuron Interface With Focused Cochlear Implant Stimulation

“…Interactions between spectral components have been well documented in studies presenting sounds simultaneously or successively (e,g., Moore, 2002;Oxenham & Shera, 2003). Likewise in electrical hearing, channel interactions have been observed in response to both simultaneous and nonsimultaneous pulse trains, the former presumably governed by the summation of current in the cochlea (Bierer 2007;de Balthasar et al, 2003;Stickney et al, 2006) and the latter by forward-masking effects in the auditory nerve and central pathways (Bierer & Faulkner, 2010;Boëx et al, 2003;Chatterjee et al, 2006;Kwon & van den Honert, 2006;Nelson et al, 2008). As in acoustic hearing, some degree of channel interaction is expected and probably desirable for discriminating complex sounds.…”

“…The larger number of active neurons would likely result in a louder percept than in Figure 1A, but it also causes partial overlap of the two neural populations. The excitation of common neural elements with dual-channel stimulation can ultimately affect perception of the separate stimulus components, a phenomenon referred to as channel interaction (Bierer, 2007;Bierer & Faulkner, 2010;Boëx, Kos, & Pelizzone, 2003;Chatterjee et al, 2006;de Balthasar et al, 2003;Kwon & van den Honert, 2006;McKay et al, 1996;Nelson et al, 2008;Stickney et al, 2006). Acoustic stimulation is generally "multichannel" in the sense that different frequency components, having different time-varying amplitudes, can fall within the passband of the same neuron.…”

Current focusing and steering: Modeling, physiology, and psychophysics

“…However these psychophysical studies also reported large variability across subjects. Some subjects exhibited forward masking patterns with a pronounced peak in a 1-2 mm region around the probe, while others had masking excitation patterns that had a broad peak extending over 8 mm or more (Boex et al, 2003;Shannon, 1998, 2006;Kwon and van den Honert, 2006a).…”

“…In general, bipolar maskers have not demonstrated narrower forward masking patterns than monopolar maskers (Boex et al, 2003;Kwon and van den Honert, 2006a;Shannon, 1983). Kwon and van den Honert (2006a) directly compared bipolar (BP + 1 and BP + 2) and monopolar configurations in four subjects.…”